1. Field of the Invention
The present invention relates to carbonaceous materials, polarizable electrodes for electrical double-layer capacitors, and electrical double-layer capacitors.
2. Prior Art
Electrical double-layer capacitors which can be charged and discharged at a large current hold considerable promise as energy storage devices for such applications as electrical cars, auxiliary power supplies and off-peak power storage. Such electrical double-layer capacitors can be more rapidly charged and discharged, and have a longer cycle life and a higher voltage durability, than lithium ion secondary cells, which have also drawn much attention lately as promising energy storage devices. On the other hand, they have a lower energy density and withstand voltage than lithium ion secondary cells. A strong need is thus felt for the development of electrical double-layer capacitors which, in addition to being capable of rapid charge and discharge and having a high durability, also have a high energy density and a high withstand voltage.
The energy stored in an electrical double-layer capacitor cell is computed as xc2xdV2 (C being the capacitance in farads (F) per cell, and V being the voltage that can be applied). Because the energy is proportional to the square of the voltage V applied, increasing the voltage that can be applied to the capacitor (withstand voltage) is an effective way to improve energy density. However, at a high voltage, the electrolyte solution decomposes, causing an increase in the internal resistance and a rapid decline in capacitance.
The polarizable electrodes used in electrical double-layer capacitors are in themselves electrochemically inert and are generally made with an activated carbon material having a large specific surface area; that is, about 1000 to 2500 m2/g. Electrodes manufacturing with an activated carbon having a large specific surface area achieve a high capacitance per unit mass of the activated carbon. However, the activated carbon has more void areas, lowering the electrode density and resulting in a lower capacitance per unit volume of activated carbon in the electrode.
Finely divided activated carbon contains numerous pores which are classified according to size as macropores (pore radius, greater than 250 xc3x85), mesopores (10 to 250 xc3x85), and micropores (4 to 10 xc3x85). This type of pore structure is believed to play a role in the large specific surface area of activated carbon.
It is also essential for the pores in finely divided activated carbon used in electrical double-layer capacitors to provide conditions suitable for approach by the electrolyte solution. Based on an investigation of the relationship between capacitance (F/cc) and the pore size distribution in activated carbon, JP-A 9-275042 discloses a good pore size distribution for activated carbon used in electrical double-layer capacitors to be one in which the most common pore size is from 10 to 20 xc3x85, and especially 13xc2x12 xc3x85.
Other efforts have focused instead on the size of the cations and anions to be adsorbed by seeking activated carbons with optimal pore sizes for this purpose and developing high-capacitance electrical double-layer capacitors using such activated carbons. For example, given that the sulfate ions commonly used in capacitors which employ an aqueous electrolyte have a size of 3 xc3x85, JP-A 10-287412 describes the use in electrical double-layer capacitor electrodes of a solid activated carbon having a pore diameter within a range of 3 to 15 xc3x85, with the volume of pores up to 15 xc3x85 in diameter accounting for 65% of the total pore volume, and having a capacitance, as measured by the constant current discharge method at 30 mA/cm2, of at least 20 F/cc.
The various ionic compounds commonly employed in electrical double-layer capacitors which use an aqueous electrolyte, such as hydrochloric acid, potassium chloride and sulfuric acid, have ion sizes of about 3 xc3x85. On this basis, JP-A 11-11921 discloses a solid activated carbon with a pore diameter of 4.5 to 15 xc3x85, which is 1.5 to 3.0 times the size of the largest ions in such electrolyte solutions.
However, unlike such electrical double-layer capacitors which use an aqueous electrolyte solution, in electrical double-layer capacitors which use a non-aqueous electrolyte solution, anion and cation movement and adsorption occur only in the presence of organic solvent molecules. It is thus essential (1) for the organic solvent molecules to fully penetrate to the interior of the activated carbon pores, and (2) for the cationic or anionic molecules to migrate through the organic solvent molecules and adsorb onto the activated carbon surface to form an electrical double layer. Thus, activated carbon used with non-aqueous electrolyte solutions must have a different pore size distribution than activated carbon used with aqueous electrolyte solutions.
For example, according to calculations based on the geometric structure of cyclic carbonate solvent molecules the distance from the hydrogen on the methyl group to the oxygen on the carbonyl group of propylene carbonate is about 8.6 xc3x85. In butylene carbonate, this distance is about 10 xc3x85. In solvent molecules having a chain-like structure, the distance between the ends of the molecule can be expected to be even larger. Moreover, although solvent molecules sometimes move individually, due to molecular interactions, they generally aggregate or form into clusters. It is thus common for such molecules to form into and move as masses larger than the calculated molecular diameter.
The smooth penetration of such gigantic solvent molecule aggregates into the activated carbon pores requires that the activated carbon contain many pores having a radius substantially larger than the solvent molecules.
Polarizable electrodes for electrical double-layer capacitors often have a potential with respect to lithium metal of about 3 V. This is because the activated carbon serving as a major component of the electrode has a potential with respect to lithium metal of about 3 V. For example, when a voltage of 4V is applied across a pair of positive and negative polarizable electrodes having a potential with respect to lithium metal of 3 V, the potential of the positive polarizable electrode with respect to lithium metal becomes 5 V and the potential of the negative polarizable electrode with respect to lithium metal becomes 1 V. The electrolyte solution thus undergoes decomposition at the positive electrode, which increases the internal resistance of the electrical double-layer capacitor and leads to a rapid decline in the capacitance.
Attempts have been made to overcome these problems by carefully studying the components (e.g., positive and negative electrodes, separator, electrolyte solution, housing) of electrical double-layer capacitors in which both the positive and negative electrodes are polarizable electrodes made primarily of activated carbon and which use a non-aqueous electrolyte solution, and trying to increase the withstand voltage per unit cell. However, the electrical double-layer capacitors achieved as a result of such efforts have a withstand voltage of about 2.5 to 3.3 V, which falls short of what is needed.
It is therefore an object of the present invention to provide a carbonaceous material which can be penetrated to the interior by a nonaqueous electrolyte solution, which has a pore size distribution optimized for the adsorption of ionic molecules onto the surface of the material and consequent formation of an electrical double layer, and which has a small specific surface area. Additional objects of the invention include providing a polarizable electrode for electric double-layer capacitors which is made using such a carbonaceous material, and a high-performance electrical double-layer capacitor endowed with a high capacitance.
As a result of extensive studies on electrical double-layer capacitors which use non-aqueous electrolyte solutions, and especially organic electrolyte solutions, as well as the relationship between the size of electrolyte solution molecules, the pore size distribution of the carbonaceous material, and the capacitance, we have found that using a carbonaceous material having a pore size distribution, as determined from a nitrogen adsorption isotherm, in which pores with a radius of up to 10 xc3x85 account for at most 70% of the total pore volume (that is, a carbonaceous material having many relatively large pores with a pore radius of more than 10 xc3x85, and especially 15 to 500 xc3x85, which allow the penetration of non-aqueous electrolyte solution molecules and are suitable for ionic molecule adsorption) allows the organic electrolyte solution molecules to penetrate smoothly to the interior of the pores, and enables the ionic molecules to adsorb to the surface of the carbonaceous material so as to form an electrical double layer. We also discovered that the use of a carbonaceous material having such a pore size distribution enables the specific surface area to be reduced to only 1 to 500 m2/g, and that the use of such a carbonaceous material in polarizable electrodes for electrical double-layer capacitors maximizes the electrode density, making it possible to achieve high-performance electrical double-layer capacitors endowed with an unprecedentedly high capacitance per unit volume.
Accordingly, in a first aspect, the invention provides a carbonaceous material having a pore size distribution, as determined from a nitrogen adsorption isotherm, in which pores with a radius of up to 10 xc3x85 account for at most 70% of the total pore volume.
In a second aspect, the invention provides a carbonaceous material having a pore size distribution, as determined from a nitrogen adsorption isotherm, in which pores with a radius of up to 10 xc3x85 account for at most 70% of the total pore volume, and having a specific surface area, as measured by the nitrogen adsorption BET method, of 1 to 500 m2/g.
Preferably, the carbonaceous materials of the first and second aspects of the invention have a pore size distribution, as determined from a nitrogen adsorption isotherm, in which at least 50% of the pores with a radius greater than 10 xc3x85 have a pore radius of 20 to 400 xc3x85.
The carbonaceous materials of the invention are typically prepared by subjecting a mesophase pitch-based carbon material, a polyacrylonitrile-based carbon material, a gas phase-grown carbon material, a rayon-based carbon material or a pitch-based carbon material to alkali activation with an alkali metal compound, then grinding the activated carbon material.
It is preferable for the carbonaceous materials to be in the form of fine particles having a cumulative average particle size after grinding of at most 20 xcexcm. It is especially preferable for the carbonaceous materials to be prepared by subjecting mesophase pitch-type carbon fibers to alkali activation, then grinding the activated fibers, and to be in the form of fine particles having a cumulative average particle size of at most 5 xcexcm.
In a third aspect, the invention provides a polarizable electrode for electrical double-layer capacitors that is made by coating a current collector with an electrode composition containing a carbonaceous material according to the first or second aspect of the invention and a polymer binder, drying the applied coat, and press-forming; which polarizable electrode has a density after drying of 0.6 to 1.2 g/cm3.
In a fourth aspect, the invention provides a polarizable electrode for electrical double-layer capacitors that is made by coating a current collector with an electrode composition containing a carbonaceous material according to the first or second aspect of the invention, a polymer binder and a conductive material, drying the applied coat, and press-forming; which polarizable electrode has a density after drying of 0.6 to 1.2 g/cm3.
The polarizable electrode of the third and fourth aspects of the invention preferably has a spontaneous potential with respect to lithium metal of at most 3.0 V.
The polymer binder in the polarizable electrode of the third and fourth aspects of the invention is preferably a polymeric material having an interpenetrating network structure or a semi-interpenetrating network structure, a fluoropolymer material, or a thermoplastic polyurethane-type polymeric material.
The polarizable electrode is typically activated by carrying out at least one charge/discharge cycle at a potential at least 30% higher than the rated potential.
In a fifth aspect, the invention provides an electrical double-layer capacitor having a pair of polarizable electrodes, a separator between the polarizable electrodes, and an electrolyte solution; wherein one or both of the pair of polarizable electrodes is a polarizable electrode according to the above-described third or fourth aspect of the invention. The electrolyte solution is preferably a non-aqueous electrolyte solution.
The electrical double-layer capacitor typically has a capacitance F1 at a high current of 90 mA/cm2 and a capacitance F2 at a low current of 1.8 mA/cm2, as measured by a constant current discharge method, such that the ratio F2/F1 is from 1 to 4.
In a sixth aspect, the invention provides an electrical double-layer capacitor having a pair of polarizable electrodes, a separator between the polarizable electrodes, and an electrolyte solution; wherein the pair of polarizable electrodes are polarizable electrodes according to the above-described third or fourth aspect of the invention, and are activated by carrying out at least one charge/discharge cycle at a potential at least 30% higher than the rated potential.
The invention as set forth above and described more fully below provides a carbonaceous material which has a small specific surface area and a pore size distribution that allows a non-aqueous electrolyte to fully penetrate to the interior thereof, and which is thus optimized for the surface adsorption of ionic molecules and the formation thereon of an electrical double layer. Electrical double-layer capacitors assembled using polarizable electrodes made with the carbonaceous material have outstanding performance characteristics, including a high voltage, a high energy density, a high capacitance and a long cycle life, in addition to which they are amenable to miniaturization. These qualities make them highly suitable for use in a broad range of applications, including backup power supplies for various types of electrical and electronic equipment such as personal computers and mobile phones, and power regeneration and storage in transport devices such as electric cars and hybrid cars.